According to the book Genesis, 70 people came down with Jacob to dwell in Egypt after contact was made with Joseph:

And the sons of Joseph who were born to him in Egypt were two souls; all the souls of the house of Jacob who came into Egypt, were threescore and ten. [46:27 KJV]

In Genesis 32:6, we're told that Esav's clan had at least 400 people in it.

Based on descriptions of the patriarchs in the Bible and our knowledge about nomadic tribes and the nomadic lifestyle, what assumptions can we make about the size of Jacob's clan before he migrates to Egypt? Should we assume that tens, hundreds or thousands of people were left behind in the land of Canaan? How might the size of Jacob's clan compare to the size of Abraham's clan?

2 Answers
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Isaac sowed in that land and reaped a hundredfold the same year. The Lord blessed him, and the man grew richer and richer until he was very wealthy: he acquired flocks and herds, and a large household, so that the Philistines envied him. And the Philistines stopped up all the wells which his father’s servants had dug in the days of his father Abraham, filling them with earth. And Abimelech said to Isaac, “Go away from us, for you have become far too big for us.”—Genesis 26:12-16 (NJPS)

From what I've read, nomadic herds are limited to about 150-200 animals per family because moving the herd from one foraging site to another is labor-intensive. If the herd's food can be supplemented with feed from agriculture, the size can be in the thousands. (See Box 1 on page 25 of "‘You can’t go home again’; Pastoralism in the new millennium" by Roger Blench.)

In Genesis, we see clans dividing because of the lack of available resources in an area:

We also see the herds are tended by both servants and children of the patriarchs. The children and (probably) the servants had families of their own, but the herds were owned by each patriarch in turn. It seems likely the property of Jacob would have split into 12 different clans, each owned by one of his sons, if Joseph were not sold to Egypt. The Nile Delta probably allowed Israel's flocks and herds to grow much larger than would have been possible in Palestine so that the people of Israel remained together.

Now the passage quoted in the question refers to Jacob's direct lineage:

All the persons belonging to Jacob who came to Egypt—his own issue, aside from the wives of Jacob’s sons—all these persons numbered 66. And Joseph’s sons who were born to him in Egypt were two in number. Thus the total of Jacob’s household who came to Egypt was seventy persons.—Genesis 46:26-27 (NJPS)

His son's wives and his servants are excluded from the count. Therefore, the actual number of people who came to Egypt could have been much larger. In fact, it would seem that none of Jacob's property was left in Palestine:

Then Joseph said to his brothers and to his father’s household, “I will go up and tell the news to Pharaoh, and say to him, ‘My brothers and my father’s household, who were in the land of Canaan, have come to me. The men are shepherds; they have always been breeders of livestock, and they have brought with them their flocks and herds and all that is theirs.’—Genesis 46:31-32 (NJPS)

Wealth in pastoral societies was measured by herd size: the larger your herd, the more wealth (in terms of labor, territory, and portable property) you controlled. Presumably the outer limit of wealth can proscribed by the latter years of Job's life:

Thus the Lord blessed the latter years of Job’s life more than the former. He had fourteen thousand sheep, six thousand camels, one thousand yoke of oxen, and one thousand she-asses.—Job 42:12 (NJPS)

The total number of animals in Job's herds was 22,000. If Job was nomadic, we'd expect his clan to total between 100 and 150 family units, but like Isaac, he was probably pastoral instead. (Job 1:13 says he used his oxen for plowing and Job 1:18-19 indicates his oldest son had a permanent house located some distance from the patriarch's dwelling.) Therefore, we can estimate that patriarch clans totaled 20 or so families, but probably fewer. Job's livestock seem to have been pastured in distant areas by animal type. It's possible they were as few as three herds plus his seven sons for a total of 10 family-sized groups.

How many people that represents is a difficult question. Jacob had 12 sons with 4 different women and we know of one daughter (Dinah). That seems like a fairly large family at the time. Making a WAG, I'd say each family represents about 5 people, which puts a patriarchal clan at 100 or so people. That squares pretty well with the number of Jacob's household in Genesis 46. But it's well short of the 400 of Esau's clan in Genesis 32:7 (NJPS):

The messengers returned to Jacob, saying, “We came to your brother Esau; he himself is coming to meet you, and there are four hundred men with him.”

One explanation might be that Esau was less settled than other Patriarchs. In that case, a total number of family units required to tend the livestock would have been an order of magnitude larger. This explanation fits with Esau's temperament, which seemed to prefer the nomadic, outdoors lifestyle. It also might be why he chose the hill country:

So Esau settled in the hill country of Seir—Esau being Edom.—Genesis 36:8 (NJPS)

Summary

The patriarchs' clans probably did not include more than a thousand people including women and servants. On the other hand, they all were probably prosperous enough to support several hundred people.

Dunbar's number

I've already answered once, but I just discovered another way to estimate the size of a cohesive social group. Robin Dunbar proposed that human neocortex size can be used to estimate the maximum size of human social groups. His estimate, which extrapolates from data gathered on other primate species, was about 150. Estimates range from 100 to 290 members. Basically, our brains can keep track of the web of relationships between that many people but no more.

Dunbar has explained [video] that an individual's social group is the people she knows well enough to trust in any social context. For many people, their list of Facebook friends is much larger than their actual social circle. Facebook (and other online communities) is designed to minimize the number of required social interactions; nobody seems to notice when I don't wish them a happy birthday. If Dunbar's theories are correct, Facebook is just the latest tool humans have developed to manage more social interactions than our physiology allows.

So how does this relate to the size of the patriarchal clans? Well, Dunbar theorizes that groups under survival pressure to stick with each other will tend to maximize group size. Over time, the standard size of a military unit. from Roman cohorts to modern companies, has hovered in this range. Any larger and cohesion begins to break down as individuals no longer comprehend the relationships within the group. When a group under pressure to maintain cohesion outgrows that limit, it will tend to split.

Every description of patriarchal clans fits the model of a cohesive social group, so we know that they could not have been much larger than 290 members and likely closer to 150. As they prospered, it was necessary to split. (See Abraham and Lot, Isaac and Abimelech, and Jacob and Esau.) According to the book of Numbers, the tribes of Israel were substantially larger than Dunbar's number. Therefore it was necessary to find other ways of maintaining social order and so Moses provided the law.